lib/util: enhanced tfork()

lib/util: enhanced tfork()

This function is a solution to the problem of fork() requiring special
preperations in the caller to handle SIGCHLD signals and to reap the
child by wait()ing for it.

Instead, tfork provides a pollable file descriptor. The caller gets the
file descriptor by calling tfork_event_fd() on the handle returned from
tfork_create() and the caller can then get the status of the child
with a call to tfork_status().

tfork avoids raising SIGCHLD signals in the caller by installing a
temporary SIGCHLD handler from inside tfork_create() and tfork_status().

The termination signal of other child processes not created with tfork()
is forwarded to the existing signal handler if any.

There's one thing this thing can't protect us against and that is if a
process installs a SIGCHLD handler from one thread while another thread
is running inside tfork_create() or tfork_status() and the signal
handler doesn't forward signals for exitted childs it didn't fork, ie
our childs.

Pair-Programmed-With: Stefan Metzmacher <metze@samba.org>

Signed-off-by: Ralph Boehme <slow@samba.org>
Signed-off-by: Stefan Metzmacher <metze@samba.org>